Vital signs of a person, for example the heart rate (HR), the respiration rate (RR) or the arterial blood oxygen saturation (SpO2), serve as indicators of the current state of a person and as powerful predictors of serious medical events. For this reason, vital signs are extensively monitored in inpatient and outpatient care settings, at home or in further health, leisure and fitness settings.
One way of measuring vital signs is plethysmography. Plethysmography generally refers to the measurement of volume changes of an organ or a body part and in particular to the detection of volume changes due to a cardio-vascular pulse wave traveling through the body of a subject with every heartbeat.
Photoplethysmography (PPG) is an optical measurement technique that evaluates a time-variant change of light reflectance or transmission of an area or volume of interest. PPG is based on the principle that blood absorbs light more than surrounding tissue, so variations in blood volume with every heart beat affect transmission or reflectance correspondingly. Besides information about the heart rate, a PPG waveform can comprise information attributable to further physiological phenomena such as the respiration. By evaluating the transmittance and/or reflectivity at different wavelengths (typically red and infrared), the blood oxygen saturation can be determined.
Conventional pulse oximeters (also called contact PPG device herein) for measuring the heart rate and the (arterial) blood oxygen saturation (also called SpO2) of a subject are attached to the skin of the subject, for instance to a fingertip, earlobe or forehead. Therefore, they are referred to as ‘contact’ PPG devices. A typical pulse oximeter comprises a red LED and an infrared LED as light sources and one photodiode for detecting light that has been transmitted through patient tissue. Commercially available pulse oximeters quickly switch between measurements at a red and an infrared wavelength and thereby measure the transmittance of the same area or volume of tissue at two different wavelengths. This is referred to as time-division-multiplexing. The transmittance over time at each wavelength gives the PPG waveforms for red and infrared wavelengths. Although contact PPG is regarded as a basically non-invasive technique, contact PPG measurement is often experienced as being unpleasant and obtrusive, since the pulse oximeter is directly attached to the subject and any cables limit the freedom to move and might hinder a workflow.
Fast and reliable detection and analysis of a pulse signal and oxygen saturation level (SpO2) is one of the most important activities in many healthcare applications, which becomes crucial if a patient is in a critical condition. In those situations, pulsatility of a heart beat signal is very weak, and therefore, the measurement is vulnerable to any sort of artifacts.
Modern photoplethysmography sensors do not always provide fast and reliable measurement in critical situations. For instance, contact finger pulse oximeters (based on transmissive PPG) are vulnerable to motion of a hand, and fails in case of centralization of a patient due to lower blood volumes on body peripherals. Contact forehead pulse oximeter sensors (using a reflective PPG measurement mode) are supposed to be more robust to a centralization effect. However, the accuracy, robustness and responsiveness of a forehead sensor depends heavily on correct positioning of a sensor on a forehead and proper pressure applied to a skin (too tight application of a sensor might reduce a local blood pulsatility, too loose application might lead to non-reliable measurements due to motion artifacts and/or venous pulsatility).
Recently, non-contact, remote PPG (rPPG) devices (also called camera rPPG devices) for unobtrusive measurements have been introduced. Remote PPG utilizes light sources or, in general radiation sources, disposed remotely from the subject of interest. Similarly, also a detector, e.g., a camera or a photo detector, can be disposed remotely from the subject of interest. Therefore, remote photoplethysmographic systems and devices are considered unobtrusive and well suited for medical as well as non-medical everyday applications. This technology particularly has distinct advantages for patients with extreme skin sensitivity requiring vital signs monitoring such as NICU patients with extremely fragile skin or premature babies.
Verkruysse et al., “Remote plethysmographic imaging using ambient light”, Optics Express, 16(26), 22 Dec. 2008, pp. 21434-21445 demonstrates that photoplethysmographic signals can be measured remotely using ambient light and a conventional consumer level video camera, using red, green and blue color channels.
Wieringa, et al., “Contactless Multiple Wavelength Photoplethysmographic Imaging: A First Step Toward “SpO2 Camera” Technology,” Ann. Biomed. Eng. 33, 1034-1041 (2005), discloses a remote PPG system for contactless imaging of arterial oxygen saturation in tissue based upon the measurement of plethysmographic signals at different wavelengths. The system comprises a monochrome CMOS-camera and a light source with LEDs of three different wavelengths. The camera sequentially acquires three movies of the subject at the three different wavelengths. The pulse rate can be determined from a movie at a single wavelength, whereas at least two movies at different wavelengths are required for determining the oxygen saturation. The measurements are performed in a darkroom, using only one wavelength at a time.
Specular reflectance of light from the skin's surface causes calibration errors leading to incorrect measurement of the concentration of various substances, such as SpO2, CO2, bilirubin, etc. in the subject's blood. Current ideas necessitate the use of polarizers in the measurement setup which are difficult to align and prove to make for a difficult setup in practice.
Further, subject motion causes an incorrect measurement of such a substance, such as SpO2, as conventionally determined. Consequently, recently proposed remote SpO2 measurement requires very stationary subjects, while contact SpO2 sensors conventionally use accelerometers to compensate for motion artifacts.
WO 01/15597 A1 discloses an imaging apparatus for representing an image of concentration ratios between a first and a second substance in a region of interest of an object, with different measuring values being represented with different colors and/or gray shades. The apparatus comprises a light source capable of irradiating the object with light, which light comprises at least three wavelengths λ1, λ2 and λ3, λ3 being an isobestic wavelength, λ1 being a wavelength at which the first substance has a lower absorption than the second substance, and λ2 being a wavelength at which the first substance has a higher absorption than the second substance. The apparatus further comprises detection means comprising a matrix of pixel detectors, for representing a virtually instantaneous image of the region of interest.
U.S. Pat. No. 7,738,935 B1 discloses methods and devices for reduction of motion-induced noise in pulse oximetry when measuring blood oxygen saturation. A portion of the light having the first wavelength, a portion of light having the second wavelength and a portion of the light having the third wavelength are received. A first signal is produced based on the received portion of light having the first wavelength. Similarly, a second signal is produced based on the received portion of light having the second wavelength, and a third signal is produced based on the received portion of light having the third wavelength. A difference between the second signal and the first signal is determined, wherein the difference signal is first plethysmography signal. Similarly, a difference is determined between the third signal and the first signal to produce a second plethysmography signal. Blood oxygen saturation is then estimated using the first and second plethysmography signals.